The "superantigens" are a class of antigen that does not obey the normal rules of MHC-restricted antigen recognition. Superantigens stimulate a large proportion of T cells through a specific interaction with the beta-chain of the T cell receptor (TCR). Thus an individual superantigen such as Mis-1 or staphylococcal enterotoxin A, when presented by MHC class II, stimulates T cells bearing specific V-beta-regions. This project aims to analyze the affinity and structure of the interaction between the TCR beta-chain and the superantigen: class II complex. The mouse mammary tumor virus (MMTV) gene products that appear to be the endogenous superantigens such as Mls will also be investigated. A secreted form of the TCR beta-chain has recently been used to demonstrate that the beta-chain is sufficient to mediate recognition of a superantigen: class II complex. The affinity and rate constants for the reaction of various V-betas with superantigens will be determined. This will allow a minimal estimate of the affinity required for a beta-chain to undergo thymic deletion or peripheral activation. This information will be extremely important in understanding the basis of tolerance induction by negative selection in the thymus. The unusual structure of V-beta3 may be related to its reactivity with a wide range of superantigens. This V-beta will be mutated to investigate the importance of these unusual structural features on superantigenreactivity and on the affinity for ligand. Comparatively little is known about the MMTV proteins that act as superantigens. They appear to be a product of an open reading frame in the proviral 3' long terminal repeat. These gene products will be investigated as to their structure and function. The genes will be expressed and the protein purified to use in in vitro studies. It will be used to determine the interaction (if it exists) with class II and V-beta. The site on the superantigen involved in these interactions will be determined. Fragments of superantigen proteins will be used to determine the region of both bacterial and endogenous superantigens that interacts with the V-beta. Fragments of the MMTV protein will be used in similar studies to investigate binding to class II. The effect on superantigen-binding of defined structural mutants of class II molecules will be investigated. The crystal structure of an endogenous superantigen complexed to class II will be determined.